The goal of this proposal is to determine how 5HT3aR-expressing interneurons regulate the structure and function of layer 5 pyramidal neurons, the main output neurons in the cortex. 5HT3aR-expressing interneurons are abundant in superficial cortical layers and consist of several distinct cell types based on their unique electrophysiological properties, morphological features and expression of molecular markers. All 5HT3aR interneurons are strongly excited by serotonin acting on 5HT3a ionotropic receptors and ACh acting on nicotinic receptors. Our preliminary studies indicate that ablation or reducing the activity of a subtype of 5HT3aR interneurons increases the dynamics and density of dendritic spines on apical dendrites of layer 5 pyramidal cells in the living cortex. To determine how different subtypes of 5HT3aR interneurons, either individually or collectively, regulate pyramidal cells' structure and function, we will first determine which subcellular domains of pyramidal cells are innervated by individual subtypes of 5HT3aR interneurons and whether such synaptic contacts are stable over time. Furthermore, using optogenefic and genetic methods to silence or activate different types of 5HT3aR interneurons, we will determine which cell type(s) are responsible for modulating structural and functional properties of layer 5 pyramidal cells in vivo. In addition, we will determine the role of 5HT3aR interneurons in regulating learning-dependent structural remodeling of layer 5 pyramidal cells. Together, our studies will provide important insights into how these distinct groups of 5HT3aR expressing interneurons target specific domains of layer 5 pyramidal neurons to control their structural plasticity and excitability, thereby contnbufing to the development and function of cortical circuits. Because dysfunction of cortical inhibition is associated with epilepsy, anxiety, depression, autism and schizophrenia, our studies may lead to a better understanding of these neurological diseases and the development of new treatment strategies.